The cardiovascular system consists of the heart, blood vessels, and blood. One of the major functions of the cardiovascular system is providing nutrients and oxygen to organs and tissues of a human body and removing metabolic wastes from the organs and tissues. During rest (low arousal condition) and stress (high arousal condition) associated with various changes in an ambient environment, the amount of nutrients and oxygen needed by tissues and organs may change. The body may adapt to the changes by adjusting the volume of blood pumped per unit of time, volume and frequency of inhaled and exhaled air, and respiration rate. The loss of capacity of the organism to adapt to the low arousal condition and the high arousal condition may be indicative of a predisposition to diseases or even an early stage of one or more diseases, and a risk of death.
Heart rate variability (HRV) may include data concerning adaptive capacity of the organism. Some current medical and fitness devices analyze the HRV in order to detect a health problem or to issue recommendations on training regimes. However, the current medical and fitness devices do not provide an accurate estimate of the risk of development of diseases, since they do not perform a rigorous analysis of the HRV. In most cases, the current analysis is limited to detection that the HRV (and other vital sign parameters) are outside pre-determined magnitude and frequency ranges obtained from population studies. The current analysis may also include determining that the HRV and other vital sign parameters are within magnitude and frequency ranges obtained in clinical studies performed without control of origin of the HRV. However, the individual features of each human organism may introduce uncertainty in the assessment of the boundaries of HRV associated with breathing, vascular, muscle, and other activities as a measure of healthy conditions. As a result, the current medical and fitness devices lack reliability of the assessment of health conditions of the human organism and prognosis of risk of heart disease, such as heart failure, atrial fibrillation, and cardiac arrest in a particular individual.